Implantable transducer devices

ABSTRACT

Receiver-stimulators comprise a nearly isotropic transducer assembly, demodulator circuitry, and at least two tissue contacting electrodes. Use of near isotropic transducers allows the devices to be implanted with less concern regarding the orientation relative to an acoustic energy source. Transducers or transducer elements having relatively small sizes, typically less than ½ the wavelength of the acoustic source, enhance isotropy. The use of single crystal piezoelectric materials enhance sensitivity.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/315,524 (Attorney Docket No. 021834-001010US), filed on Dec. 21,2005, and claims the benefit of the following provisional applications:60/639,027 (Attorney Docket No. 021834-000800US), filed on Dec. 21,2004; 60/689,606 (Attorney Docket No. 021834-000810US), filed on Jun. 9,2005; and 60/639,056 (Attorney Docket No. 021834-001000US), filed onDec. 21, 2004. The full disclosures of each of these prior filings areincorporated herein by reference.

The subject matter of this application is related to that of thefollowing commonly owned patent applications: Ser. No. 10/869,242(Attorney Docket No. 021834-000310US); Ser. No. 10/869,776 (AttorneyDocket No. 021834-000130US); and Ser. No. 10/869,705 (Attorney DocketNo. 021834-000620US). The full disclosures of each of these priorfilings are incorporated herein by reference but the benefit of thefiling dates is not being claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The stimulation of cardiac tissue using an acoustic transducer, referredto as a controller-transmitter, and one or more implantedreceiver-stimulator devices has recently been proposed by the inventorsherein in the patent applications referred to above. Thecontroller-transmitter produces an acoustic signal which is received bythe receiver-stimulator, and the receiver-stimulator in turn generatesan electrical signal which is delivered to cardiac or other tissuethrough coupled tissue electrodes. The controller-transmitter may beexternal, but will usually be implanted, requiring that thecontroller-transmitter have a reasonable size, similar to that ofimplantable pacemakers, and that the controller-transmitter be capableof operating from batteries for a lengthy period, typically three ormore years. The relatively small size and relatively long operationalperiod require that the receiver-stimulators efficiently utilize theacoustic energy from the controller-transmitters.

For those reasons, it would be desirable to provide implantabletransducer devices which are able to efficiently receive acoustic energyfrom implanted or external acoustic transmitters. It would beparticularly desirable if the transducers could operate in an isotropicor nearly isotropic fashion where they could efficiently receiveacoustic energy from an acoustic transmitter regardless of the relativeorientation between the transmitter and the implanted transducer. Atleast some of these objectives will be met by the inventions describedhereinafter.

2. Description of the Background Art

The following patents and patent publications describe variousimplantable transducers capable of converting applied acoustic energyinto an electrical output: U.S. Pat. Nos. 3,659,615; 3,735,756;5,193,539; 6,654,638; 6,628,989; and 6,764,446; U.S. Patent ApplicationPublications 2002/0077673; 2004/0172083; and 2004/0204744; and publishedGerman application DE 4330680.

BRIEF SUMMARY OF THE INVENTION

Systems and methods are provided for delivering electrical energy tobody tissues for a variety of purposes. The energy will typically bedelivered in order to stimulate cardiac tissue, for example in cardiacpacing for bradycardia, for termination of tachyarrhythmia, forbi-ventricular resynchronization therapy for heart failure, or the like.The systems and methods of the present invention, however, could be usedin a variety of other applications, including applications for nervestimulation, brain stimulation, voluntary muscle stimulation, gastricstimulation, bone growth stimulation, pain amelioration, and the like.

In a first aspect, the present invention provides an implantablereceiver-stimulator device which is capable of receiving acoustic energydelivered from an acoustic source (physically separate from thereceiver-stimulator device) and converting that acoustic energy to anelectrical signal. The receiver-stimulator of the present invention willusually be very sensitive and will usually be able to receive andconvert low levels of acoustic energy to produce electrical signalswhich are able to stimulate myocardial tissue. Typically, with devicesof the present invention with cross sectional areas on the order of 3mm², an acoustic wave having a pressure level in the range from 0.2 to0.4 mega Pascals (an intensity level of 1.3 to 5.6 W/cm²), can beconverted to electrical signals in the range from 1.0 to 2.0_Volts.Thus, the devices of the present invention will usually be veryefficient and capable of converting a large portion of the receivedacoustic energy into electrical energy, typically with a conversionefficiency of at least 25%, often being at least 50%. In addition tosuch high sensitivity and efficiency, the implantablereceiver-stimulators of the present invention are also capable offunctioning at least substantially isotropically. That is, the devicesensitivity will be isotropic. By “isotropic,” it is meant that thereceiver-stimulator will have a transducer assembly capable of receivingacoustic energy in a manner which is substantially insensitive to therelative orientation of the device to the acoustic source. The electricsignal produced by the receiver-stimulator device in response toincident acoustic energy will vary by no more than ±6 dB as theorientation of the device varies relative to the acoustic source, oftenvarying by no more than ±3 dB, preferably varying by no more than ±1 dB.

In a first specific embodiment, an implantable receiver-stimulatorcomprises a transducer assembly, typically being capable of isotropicoperation as noted above, which receives acoustic energy from anacoustic source and which produces an electrical signal in response tothe acoustic energy. The device further comprises demodulator circuitrywhich receives the electrical signal and which produces a biologicallystimulating electrical output, e.g., suitable for cardiac pacing, nervestimulation, brain stimulation, voluntary muscle stimulation, painamelioration, or the like. The device will further include at least twotissue-contacting electrodes which are coupled to the demodulatorcircuitry to receive the stimulating electrical output and deliver saidoutput to the tissue. Either or both of the electrodes may be mounteddirectly on the device, in some instances forming a portion of thedevice casing, or may alternatively be connected to the device by wires,cables, or the like, for placement.

The transducer assembly may comprise a cylindrical piezoelectrictransducer having a pair of electrodes formed over opposed surfacesthereof. The incident acoustic energy will cause the piezoelectrictransducer to vibrate and generate electrical charge which is collectedby the electrodes and available for delivery to the demodulatorcircuitry. In a first exemplary embodiment, the piezoelectric transducermay be composed of a polycrystalline ceramic piezoelectric material.When the ceramic piezoelectric material is formed in the shape of atube, the opposed electrodes may typically be formed over the outer andinner cylindrical surfaces of the transducer although electrodes overthe opposing flat end surfaces may also be used.

In a preferred exemplary embodiment, however, the piezoelectrictransducer will be composed of a single crystal material, typicallybeing cut in the <001> orientation. A preferred single crystal materialcomprises PMN-xPT material, where x is in the range from 5% to 50% byweight. Other single crystal materials may be of the compositionPZN-xPT, or Relaxor-PT materials. When the piezoelectric transducer iscomposed of a single crystal, the opposed electrodes are preferablyformed over the opposed flat end surfaces of the cylinder, not thecylindrical surfaces. Alternatively, for alternate crystal planes,electrodes formed on cylindrical surfaces or cylindrical surfaces onsectioned and composite crystal assemblies, may be preferred.

In a still further embodiment of the implantable receiver-stimulator ofthe present invention, the transducer assembly comprises a plurality ofindividual transducer elements. The demodulator circuitry similarlycomprises a plurality of individual demodulator circuits, and each ofthe transducer elements is attached to one of the individual demodulatorcircuits. The transducer elements themselves will typically have amaximum dimension which is approximately one-half wavelength of theexpected acoustic transmission, but the cumulative lateral dimensions ofthe individual transducer elements will preferably be much greater thana single wavelength. On the output of the demodulator circuitry therewill be provisions for summing the electrical signals from each of theindividual demodulator circuits to produce the biologically stimulatingelectrical output. Electrical signals may be summed in parallel, inseries, or in a series-parallel combination.

In a second aspect of the present invention, methods for deliveringenergy to an implanted receiver-stimulator comprise implanting areceiver-stimulator, typically formed as an assembly having a transduceror transducers, being substantially isotropic as described above inconnection with the devices of the present invention. Acoustic energy isdirected to the implanted receiver-stimulator assembly from an acousticsource, which may be implanted or located externally, and thetransducers produce electrical signals which vary by no more than ±6 dBas the orientation of the transducers vary relative to that of theacoustic source. The electrical signal is demodulated to produce abiologically stimulating electrical output, and the electrical output isdelivered to tissue. The acoustic energy may be delivered to thereceiver-stimulator from an external source, but will preferably bedelivered from an implanted acoustic source. The electrical outputflowing between stimulating electrodes which are in contact with tissuemay possess specific characteristics of voltage, current, waveform, andthe like. These electrical characteristics will be selected to stimulatethe target cardiac tissue, nerve tissue, brain tissue, voluntary muscletissue, bone tissue, or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a receiver-stimulator constructedin accordance with the principles of the present invention.

FIG. 2 illustrates a first exemplary transducer design of a type usefulin the receiver-stimulators of the present invention.

FIG. 3 illustrates a second exemplary transducer design of the typeuseful in the receiver-stimulators of the present invention,particularly being useful with single crystal transducers.

FIG. 4 is a schematic illustration of the elevation angle definition forbeam profile measurement.

FIGS. 5 and 6 illustrate the elevation angle beam profiles for a singlecrystal transducer and a polycrystalline ceramic transducer.

FIGS. 7-9 are impedance plots for different transducer materials.

FIG. 10 is an embodiment of transducer assemblies employing multipletransducer elements

FIGS. 11-12 are two embodiments of transducer assemblies employingmultiple transducer elements in accordance with the principles of thepresent invention.

FIG. 13 illustrates an exemplary construction for a transducer assemblyhaving multiple individual transducer elements.

FIGS. 14A-14I show exemplary waveform summation plots produced by themultiple element transducer assemblies of the present invention.

FIGS. 15A-15 d illustrate various single crystal orientations andrespective sensitivity profiles of transducer in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1, an exemplary receiver-stimulator 10constructed in accordance with the principles of the present inventioncomprises a transducer assembly 12, demodulator circuitry including arectifier circuit 13 and a filter circuit 14, and tissue-contactingelectrodes 15. Optionally, impedance matching circuit 16 may be providedto match the output of the transducer assembly with the electricalimpedance presented by the rectifier/filter circuits and the tissuecontacted by the electrodes. The present invention provides certainspecific designs for the transducer assembly 12 and for assemblies oftransducers which in turn provide a highly isotropic receiver-stimulatoroperation such that the output of the transducer or transducers ishighly independent of the orientation of the transducer or transducersrelative to an acoustic source.

A first nearly isotropic transducer assembly 29 useful in the presentinvention is illustrated in FIG. 2. The transducer assembly 29 generallycomprises a cylindrical polycrystalline piezoelectric tube 20 with anouter cylindrical electrode 21 and an inner cylindrical electrode 22.Typically the ceramic may be mounted on a thin walled tube 23 ofpolyamide or other suitable material, with or without an epoxy bondbetween the inner wall of the ceramic and the outer wall of the tube.Both flat faces of the ceramic may have an acoustic absorber 24 betweenthe ceramic and before the start of the structural body 25. All internalsurfaces are typically bonded with low viscosity epoxy. The outersurface of the assembly is typically coated with a low viscosity epoxyunder a thin walled shrink tube 26. Prior to assembly, electrical leads28 are soldered to the inner and outer electrodes, and passed throughthe hollow inner volume 27 of the inner tube. Optionally, other circuitcomponents as shown in FIG. 1 may also be positioned in the hollow innervolume 27. Devices of this type can provide near isotropic sensitivity,typically on the order of 6-12 dB down, in the elevation angle along thecylindrical axis as compared to the sensitivity normal to thecylindrical axis, where the elevation angle with respect to thecylindrical axis is depicted in FIG. 4.

While the transducer assembly 29 of FIG. 2 provides a high degree ofisotropic sensitivity, it would be desirable to provide transducerassemblies which operate even more isotropically. Referring to FIG. 3,the use of single crystal piezoelectric materials can provide furtherenhancement. A single crystal device 39 would be fabricated in a similarmanner to the polycrystalline ceramic device, with the exception thatthe electrodes 31 and 32 of the single crystal piezoelectric would be onthe flat faces of the cylinder 30 which is a single crystal material cutin the <001> orientation, and wired accordingly with leads 28. Standardcrystallographic terms are used here to define crystal axes. The Millerindices (hkl) define the orientation of a plane with respect to thecrystal axes. A separate axis perpendicular to the plane (hkl) isdefined by the term <hkl>. Therefore, a (001) plane would be parallel tothe xy crystal plane and perpendicular to the crystal z axis. The <001>axis is thus parallel to the crystal z axis.

Devices of the type depicted in FIG. 3 have been fabricated with singlecrystal component sizes virtually identical to the ceramic components.As detailed below, there are similarities in performance such as beamprofiles. Most significantly, the single crystals have superiorisotropic sensitivity, especially at a low frequency resonance which isnot achieved by the higher frequency resonance of the ceramicpiezoelectrics.

FIGS. 5 and 6 depict measured and model data for beam profiles at 340kHz and 550 kHz of representative devices. The single crystal deviceprofiles are depicted in FIG. 5 and the ceramic device profiles aredepicted FIG. 6. For these profiles, the beam angle is defined as theelevation angle as depicted in FIG. 4. An elevation angle of 0 degreescorresponds to an incident beam normal to the cylindrical axis of thedevice, 90 degrees parallel to the cylindrical axis, and 180 degreesnormal to the cylindrical axis, but in the opposite direction from 0degrees. Due to symmetry, in the absence of gross fabrication defects,these devices will usually have sensitivity fluctuations of less than 1dB in azimuth, around the cylindrical axis of the piezoelectric tube.With respect to the elevation angle, a perfectly isotropic device wouldhave a flat response from 0 to 90 to 180 degrees. Further, at lowerfrequencies both devices exhibit less anisotropy than predicted bydiffraction theory. However, the variation in anisotropy remainsinconsistent with a simple diffraction model based on device size andfrequency. Acoustic coupling through structural components of the testarticles are assumed responsible for this discrepancy.

With respect to sensitivity, Table 1 below summarizes device sensitivityin a plane normal to the cylindrical axes of the devices. The deviceswere exposed to long bursts of ultrasound, in the general range of 340kHz and 600 kHz. With no electrical load on the devices, the generatedpeak-to-peak voltage was measured by a high input impedance digitaloscilloscope, with the results tabulated below with respect to theultrasound field strength in MI (Mechanical Index, defined asrarefactional pressure in mega Pascals divided by the square root of thefrequency in mega Hertz). Subsequently, in each test, the devices wereelectrically loaded with a 500 ohm resistor, with the results nexttabulated below with respect to field strength. The electrical impedanceof 500 ohms was used and was representative of the impedance betweenelectrodes in contact with various human tissue types. And lastly, thedevices were near optimally impedance matched with a transformer,connected to a full wave rectifier with a 0.1 micro Farad capacitor, andloaded with 500 ohms. The peak amplitude of the resulting DC (directcurrent) voltage envelope with respect to the field strength is reportedbelow.

TABLE 1 Unloaded 500 Ω Load Det/Fil Device Frequency Vac/MI Vac/MIVdc/MI s/n Type (kHz) (Vpp) (Vpp) (Vop) 2199 Ceramic 550 8.90 5.40 2.052199 Ceramic 340 4.28 2.73 0.65 2216A Crystal 570 30.4 8.53 4.06 2216ACrystal 340 102.0 16.8 8.10 2195 Ceramic 600 17.0 10.9 2.76 2195 Ceramic340 3.70 1.97 0.41

In the above Table, device 2199 was a PZT-5H ceramic tube, 0.070 incheslong, 0.070 inches in outside diameter, and 0.045 inches on innerdiameter. The tube was polarized with electrodes on the inner and outercylindrical surface. Device 2216A was a PMN-32% PT single crystal tube,with the same dimensions as device 2199. However, the device was cutfrom a plate perpendicular to the <001> crystal orientation axis. Theelectrodes were on the flat faces with polarization between theelectrodes. Device 2195 was a ceramic tube in all aspects the same asdevice 2199, with the exception of a 0.040 inch length.

In an open circuit mode, the single crystal devices are vastly superiorto the ceramic devices. However, this represents an unrealisticsituation in that sufficient current needs to be derived from thedevices to stimulate tissue. While the single crystal materials stillenjoy a substantial advantage when loaded with a representative tissueimpedance, the performance gap has lessened. And lastly, when drivingimpedance matching, rectifier, filter, and representative tissue loadedcircuits, there was still a good performance gap, but with slightvariations.

The primary reason for the excellent performance of the single crystalmaterial was the low frequency constant which results in a resonance atapproximately 340 kHz, as seen in FIG. 7. For the same sized piece ofmaterial, there exists a small resonance in the ceramic at approximately600 kHz as seen in FIG. 8, thus slightly enhanced relative to theperformance of the ceramic at 550 kHz. A much stronger resonance existsat approximately 900 kHz. This resonance unfortunately cannot be used asthe wavelength thus becomes smaller than the device, predisposing thedevice to yet greater elevation angle anisotropy. Interestingly, indevice 2195, with a length half that of 2199, the length mode resonancewas pulled down to approximately 600 kHz as seen in FIG. 9, and thus theslightly better performance of the shorter device at 600 kHz wasobserved.

In comparing the performance of the single crystal at its resonance of340 kHz with the ceramics at their lowest frequency resonances at 550 to600 kHz, the single crystal device was still on average more than 10 dBmore sensitive, fully loaded, than the ceramic devices. Comparing thesingle crystal at its off resonance frequency with the ceramics at theiroff resonance frequencies, the single crystal device was more than 17 dBmore sensitive. The single crystal material was thus seen to offersignificant improvement over the ceramic material as a source ofelectrical energy in an acoustic field for the tissue stimulator.

The present invention has detailed the implementation of single crystalpiezoelectric tubes cut in the <001> orientation for use in implantablereceiver-stimulator devices, where the sensitivity normal to the crystalaxis is circumferentially uniform in all directions, as depicted in FIG.15A by the dashed line around the tube cross sectional view. Multipledifferent orientation planes of single crystals also have potentialutility. One such possible configuration would utilize the properties ofthe single crystal tube cut in the <011> plane, where the lateralsensitivity has a crossed dipole shape, with an amplitude in the <100>direction higher by a factor of approximately two than the singlecrystals discussed above, but with an orthogonal sensitivity being ofreverse polarity and less amplitude, as depicted in FIG. 15B. If thiscrystal were to be directly implemented, the reverse polaritysensitivity would detract from the stronger orthogonal dipole lobes, andyield a device with perhaps less sensitivity than the devices asdescribed above. However, if quadrants of the single crystal comprisingthe higher sensitivity dipole lobes (as highlighted by the arcs in FIG.15B) were to be cut out, and four quadrants of this high sensitivitymaterial reassembled as depicted in FIG. 15C, the net effect would be asingle crystal tube with high sensitivity in all four quadrants, and netgreatly increased sensitivity. Since the original tubular shape has beenmaintained, all mechanical resonances within the device remain the same.Alternatively, the cut sections with favorable crystal orientation maybe smaller, 60 degrees of arc instead of 90 degrees of arc, andreassembled, as depicted in FIG. 15D, for yet higher sensitivity. Sincethe anticipated operating wavelength is larger by approximately a factorof two than the physical size of the reassembled device, the sensitivitywill be averaged over the surface of the device.

A further embodiment of the implantable receiver-stimulators of thepresent invention utilizes a transducer assembly which includes multipletransducer elements at least some of which have a size (equal to or lessthan one-half wavelength) selected to enhance the isotropic nature ofthe individual elements. Generally if the piezoelectric transducer sizeexceeds one half wavelength of the acoustic signal, directionalvariations in sensitivity will begin to dominate performance, whereaswith device sizes less than one half wavelength, device sensitivityapproaches isotropy, being almost uniform in all directions.Hydrophones, which are devices to sample acoustic fields, typically haveupper operational limits corresponding to a sensor size of one halfwavelength. Larger sizes would be preferred for hydrophone elements asthe output is directly proportional to the cross sectional area of thedevice. Thus, for the receiver-stimulator, multiple elements are addedspecifically to increase the cross sectional area of the device toincrease sensitivity. The implantable devices must have near isotropicresponses, as the orientation of a device with respect to the acousticexcitation field may not coincide with the orientation of the device asit is implanted in the tissue which requires stimulation.

As depicted in FIG. 10, a transducer assembly featuring multipleelements 112 a, 112 b, 112 c, 112 n and output leads 113 a, 114 a, 113b, 114 b, 113 n, 114 n are wired in parallel to a single pair ofconnecting leads 116 a and 116 b to a single detector/filter circuit118, which passes the combined signal to tissue contact stimulationelectrodes 120 a and 120 b, via leads 119 a and 119 b. This technique,however, while offering more ceramic volume to achieve a greater poweroutput, is subject to the constructive or destructive interference ofsignals into the detector/filter 118 from each of the individualelements due to the different arrival times of the signals from eachelement. It would be difficult to position the set of individualelements in such a manner as to assure only constructive interference ofthe individual signals. This configuration offers high sensitivity onlywhen the phase of the incident acoustic signal is identical at eachelement and thus is not a preferred embodiment of the present invention.

In order to overcome this deficit and yet to use multiple elements forgreater output power, and to reduce or eliminate constructive anddestructive interference between the elements, a separatedetector/filter for each element is provided. The combination of asingle transducer element and a single detector/filter may be referredto as a channel. As depicted in FIG. 11, individual elements 122 a, 122b, 122 c, 122 n drive their own respective detector/filters 125 a, 125b, 125 c, 125 n via their leads 123 a, 124 a, 123 b, 124 b, 123 n, 124n. The output leads 126 a, 127 a, 126 b, 127 b, 126 n, 127 n of thedetector/filters are wired in parallel via leads 129 a and 129 b and arethence connected directly to the stimulation electrodes 130 a and 130 b.In this manner, the current delivery of the device (assuming theelements are all the same and receive the same acoustic signal) ismultiplied by the number n of individual channels within the device.

Alternatively, as depicted in FIG. 12, the outputs of the individualdetector/filters can be wired in series, with output leads 139 a and 139b passing directly to the tissue stimulation electrodes 140 a and 140 b,while intermediate leads such as 137 a and 136 b between channels areconnected. In this manner, the voltage delivery of the device (assumingthe elements are all the same and receive the same acoustic signal) ismultiplied by the number n of individual channels within the device.

Further, it is also possible to combine series and parallel connectionson the outputs of the individual channels to achieve at least partiallyspecific impedance matching between the individual piezoelectricelements and a target tissue mass.

To demonstrate this concept, a multi-element section 142 was fabricatedas depicted in FIG. 13, comprising four individual cylindrical tubetransducer elements 141 a to 141 d, which were epoxy bonded to theinside wall of thin walled polyimide tubing 143. Transducer electrodelead wires 144 a to 144 h were fed to one end of the tubing. There wasno fluid path to the hollow air-filled inside of the polyimide tube. Theindividual transducer elements had an outer diameter of approximately0.6 wavelengths and a length of approximately 0.8 wavelengths, with agap between elements of approximately 0.6 wavelengths at 500 kHz. Inthis prototype implementation, the detector/filter circuits were voltagedoublers, as described in a co-pending application.

In FIGS. 14A and 14B the traces from top to bottom follow the top tobottom order of the index to the right of the chart. FIG. 14A depictsthe output voltage from each of the four elements (top four traces) whenthe device was oriented askew with respect to an ultrasound beam.Because of the random phase variations among the elements, a seriesconnection (RF SUM) resulted in a net signal weaker than that from anyelement on its own. Indeed, if the outputs of the individual elementswere to be mathematically summed (MATH SUM), the waveform is virtuallyidentical to that of the RF SUM. Even though each individual element mayhave had a near isotropic response, destructive interference of theseries-connected individual element signals has given a diminishedoutput. The slight variations in the amplitudes of the four elements isdue to slight fabrication variations and possible interaction of theacoustic beam between the elements.

FIG. 14B depicts a more ideal case, where the four elements are alignedvirtually perpendicular to the ultrasonic beam. Since the size of theindividual elements was slightly greater then the nominal halfwavelength rule recited above, there is a slightly greater output of theindividual element (top four traces) in this case, than in the previous“random alignment” angled case. In the same manner as above, the RF SUMrepresents the hardware wired combination of devices compared to themathematical combination, MATH SUM. Note however that the RF SUM isslightly lower in amplitude than the MATH SUM. During the measurements,the individual elements were connected to individual open endeddetector/filter circuits which presented a different electrical load aswhen the elements where wired in series and connected to a single openended detector/filter circuit. Note further that in stark contrast toFIG. 14A, the phases of channel 1 to channel 4 are precisely aligned.

When looking at the output of the detector/filter circuit under a noload condition, the output voltage falls off very slowly as compared tothe voltage rise due to the input signal from the transducer element.This is seen in the lower four traces of FIG. 14C for the four channels(transducer element and detector/filter), where from approximately −5microseconds to 100 microseconds the individual transducer elements areexposed to an ultrasonic field. In this case, and for all measurementsof the remaining figures, the orientation of the four transducers withrespect to the acoustic beam is skewed, causing random phases for theoutput of the individual transducer elements, as was the case for FIG.14A. In FIGS. 14C and 14D the top two traces represent the hardwaresummation of the DC output of the detector/filters, and the mathematicalsum of the individual outputs. Note that the actual hardwired summationis virtually identical to the mathematical sum of the individualchannels. This is in stark contrast to the hardwired case of FIG. 14A,where hardwired summation was performed on the output of the individualtransducer elements with a single detector/filter.

FIG. 14D represents the same conditions as FIG. 14C, except the outputsof the individual channels, comprising the individual transducerelements and their respective detector/filter circuits, were loaded with1000 ohms, and the hardwired post detector summation was loaded with4000 ohms. With 4000 ohms on the summed output, the current deliveryfrom each channel was the same as in the unsummed state. Indeed, FIG.14E represents the same scenario wherein the hardwired output is loadedwith 1000 ohms, which placed a factor of four higher current demand oneach of the four channels, and which consequently resulted in asignificantly lower amplitude DC SUM since the individual transducerelements were unable to meet the demand. Further, the waveform of thesummed output has become markedly flattened, clearly demonstrating thecurrent threshold of the elements. Note also that in this case ofresistive loading on the individual channels and on the summed outputs,while the rise time of the detector/filter circuit is the same asbefore, the fall time is reduced, due to the drain of charge off thefilter capacitors.

FIGS. 14G and 14F represent the same conditions as FIGS. 14C and 14D,with the exception that the channels were wired in parallel at thechannel outputs. In both the no load case (FIG. 14G) and in theresistively loaded case (FIG. 14F), the same minor variation in outputsfrom the different channels were observed. However, the hardwiring inparallel has created an average voltage, DC SUM, as corroborated by themathematical average, MATH AVE, of the four traces. In the electricalloading case, 1000 ohms was applied to outputs in the individualdetector/filter circuits, and 250 ohms was applied to the hardwiredparallel sum. The reduction in resistance had the effect of keeping thevoltage constant. The combined device achieved a four times greatercurrent delivery capability.

Lastly, in FIGS. 14H and 14I, the outputs of the detector/filtercircuits were hardwired together, from one to four elements at a time,either in series or parallel, respectively. In FIG. 14H, adding elementssimply raised the output voltage in direct relation to the number ofchannels. In FIG. 14I, the output voltage remained generally constantwith the addition of channels, but the current delivery capabilityincreased according to the number of channels (no current graphicshown).

It has thus been demonstrated experimentally that summation in the RFdomain (transducer element output) will result in constructive anddestructive interference, dependent on the phase relationship betweenthe elements. Alternatively, by providing summation after detection andfiltering, a phase independent environment is established, with onlyconstructive interference.

The detector circuits discussed for this application might include halfwave rectifiers, full wave rectifiers, voltage doublers, charge-pumpdevices, and the like. Filters may include series inductors, parallelcapacitors, combinations of the same, and the like. Impedance matchingmay be accomplished through transformer devices, active or passivecircuit components, or may be incorporated into the design of thedetector/filter circuits. Further details on the detector/filtercircuits are provided in co-pending application.

With the requirement for isotropic transducers in thereceiver-stimulator, transducer size (in all lateral dimensions) shouldnot exceed approximately 0.5 wavelengths, subject only to the amount ofvariation tolerated in signal strength at various elevation angles.Given a velocity of sound in normal tissue of approximately 1.5millimeters per microseconds, at 1 MHz device sizes shall not exceedapproximately 0.75 millimeters, at 0.5 MHz 1.5 millimeters, and at 250kHz 3.0 millimeters.

Transducers in the receiver-stimulator can be positioned and located inany orientation, with respect to the axis of the transmitted acousticbeam. Transducers can be mounted in a linear manner as depicted in FIG.13, or they can be mounted in a star pattern, a circular pattern, or across pattern, to recite just a few options. Further, the angularorientation of any transducer within the receiver-stimulator can berandom. Anatomical conditions can be allowed to dictate the orientationof the implantable device.

While the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications, and equivalentsmay be used. Therefore, the above description should not be taken aslimiting the scope of the invention which is defined by the appendedclaims.

1. An implantable receiver-stimulator device: a transducer assemblywhich receives acoustic energy from an acoustic transmitter and produceselectrical energy in response to the acoustic energy, wherein theelectrical energy is characterized by a source impedance; circuitrywhich receives the electrical energy and produces a biologicallystimulating electrical output; and at least two tissue contactingelectrodes which receive the stimulating electrical output and deliverelectrical energy to tissue, characterized by a load impedance, whereinthe source impedance and the load impedance are matched.
 2. The deviceof claim 1, wherein the impedance matching is achieved using ademodulator circuitry.
 3. The device of claim 2, wherein the demodulatorcircuitry is configured for a 40-100% match between the source impedanceand the load impedance.
 4. The device of claim 3, wherein the match is100%.
 5. The device of claim 2, wherein the transducer assemblycomprises a plurality of individual transducer elements and wherein thedemodulator circuitry further comprises a plurality of individualdemodulator circuits, which the electrical signal from each transducerelement fed to the demodulator circuit which produces a demodulatoroutput, further comprising a summing circuitry which sum all of thedemodulator outputs from all of the demodulator circuits to produce thebiologically stimulating electrical output.
 6. The device of claim 5,wherein the demodulator outputs are summed in parallel.
 7. The device ofclaim 5, wherein the demodulator outputs are summed in parallel.
 8. Thedevice of claim 5, wherein the demodulator outputs are summed in series.9. The device of claim 5, wherein the demodulator outputs are summed ina series-parallel combination.
 10. The device of claim 5, wherein thetransducer elements each have a size less than one-half the wavelengthof an acoustic source while collectively having a size greater than onwavelength in any lateral dimension.
 11. The device of claim 5, whereinthe source and load impedances are matched using a transformer between atransducer element and a demodulator circuit.
 12. The device of claim 5,wherein the source and load impedances are matched using a transformerbetween one or more demodulator circuits and the tissue electrodes. 13.The device of claim 1, wherein the transducer assembly is composed ofpiezoelectric transducer material with specific impedancecharacteristics.
 14. The device of claim 13, comprising a shell having awall surrounding an interior volume, wherein the transducer assembly andcircuitry are within the interior volume and wherein at least a portionof the wall is acoustically transmissive.
 15. The device of claim 14,wherein the entire shell is acoustically transmissive.
 16. The device ofclaim 15, wherein the shell is cylindrical.
 17. The device of claim 16,wherein the shell has a diameter less than 5.0 mm and a length less than20.0 mm.
 18. The device of claim 17, wherein the shell has a diameterless than 3.0 mm and a length less than 10.0 mm.
 19. The device of claim1, wherein the source and load impedances are matched using atransformer between the transducer and the circuitry.